1. Field of the Invention
The present invention is related to microfluidic devices, and specifically to methods for modifying the conductivity of materials used in the fabrication of those devices.
2. Related Art
Microfluidic technology enables the miniaturization and automation of many laboratory processes. Devices employing microfluidic technology can integrate the power of an entire laboratory full of equipment and people into a single “lab-on-a-chip.” Each microfluidic device (hereafter also referred to as a “chip”) contains a network of microscopic channels, or microchannels, through which fluids can be moved and in which experiments can be performed. The design of microfluidic devices for biochemical applications involves the disciplines of fluid dynamics, biochemistry, software, and thin film manufacturing.
In microfluidic devices, the driving forces that move fluids within the channels of the device can be electrokinetic forces, pressure forces, or a combination of the two. Electrokinetic forces are typically generated by applying an electric field across a microchannel, where the direction of the field is parallel to the desired direction of fluid flow. The electric field is typically applied by placing electrodes in reservoirs at the ends of the microchannel, and applying a voltage across the electrodes with a computer-controlled power supply. The voltage applied across the electrodes produces fluid flow via one or both of the phenomena of electroosmosis or electrophoresis. Electroosmosis occurs when an electric field is applied across a channel whose surface or walls contain charged functional groups. The charge on the channel wall ionizes a thin layer of fluid near the wall. This thin layer of ionized fluid is attracted to one of the electrodes, creating a flow of ionized species toward that electrode. The flow of ionized species produces both a bulk fluid flow and an electrical current. The bulk flow rate through a microchannel can be controlled with a high degree of precision by controlling the electrical current that accompanies the flow through the microchannel. The other phenomena that produces electrokinetic flow, electrophoresis, is the movement of charged molecules or particles in a fluid subjected to an electric field. Electrophoresis can be used to move charged molecules in solution, or to separate charged molecules that have different electrophoretic mobilities (which is roughly their charge to mass ratio). Electrophoresis and electroosmosis often occur at the same time when an electric field is applied to a microchannel. Techniques have been developed for minimizing one electrokinetic force while maintaining the other, as appropriate, for a given application. Precise control over fluid flow within microchannels requires precise control of the driving forces, such as electrokinetic or pressure forces. Precise control over fluid flow also requires precise engineering of the microchannels themselves because fluid flow also depends on channel geometry and surface properties.
Microfluidic devices are typically fabricated by etching or embossing grooves into a substrate, and then affixing a cover to the substrate to form the microchannels. In most microfluidic devices that employ electrokinetic flow, both the substrate and the cover plate are made of an insulating material such as glass. Insulating materials help reduce the electrical current leakage between microchannels. By reducing current leakage between microchannels, the use of insulating materials allows an increased packing density of components, such as microchannels, in a microfluidic device.
In some applications, it may be advantageous to allow a localized leakage of current between different channels in a microfluidic device. The leakage of current between channels allows the electrical currents that drive electrokinetic flow to flow in directions other than parallel to the length of the microchannels. In other words, having a conductive path between channels provides the ability of initiating electrokinetic flow in directions other than along the length of a channel. For example, fluid could be made to flow into the sidewall of a channel. Microfluidic devices with a conductive path between channels could provide advantages over standard microfluidic devices in the areas of sample concentration and two-dimensional separation.
One set of researchers has fabricated microfluidic devices that employ electrical current leakage between microchannels for the purpose of concentrating samples. Khandurina, J., et al., Anal. Chem. 71, pp. 1815-1819 (1999). In these microfluidic devices, the current leaks between microchannels through a porous membrane. The porous membrane is a separate layer of material sandwiched between the cover plate and substrate of a microfluidic device. In the devices shown in Khandurina, fluid from a main channel that terminates at a “T” shaped intersection with a separation channel is made to flow straight into the opposing wall of the “T” shaped intersection by allowing electrical current to flow into the opposing wall through a porous membrane above the wall. By flowing sample from the main channel into the opposing wall, the sample accumulates, and thus concentrates, at the “T” intersection. When enough sample has accumulated at the intersection, the sample is directed to flow down the separation channel. The device in Khandurina could be useful in assays in which a sample to be separated into components must be concentrated in order to increase the concentration of at least some of the components above a detectable threshold.
There are several problems with microfluidic devices that employ porous membranes to provide conductive paths between microchannels. First, the lifetime of these devices is short and unpredictable due to the nature of the porous membrane. Second, the resistance of the porous membranes may change with time. Third, the process for fabricating porous membranes lacks the dimensional control needed to fabricate porous membranes between closely spaced microchannels. Fourth, the nature of the conductivity of the porous membrane is not certain, and that could lead to unexpected fluctuations of conductivity both between and within microfluidic devices. Finally, having a conductive path between microchannels may prevent the manufacture of devices with densely packed microchannels.
Given the limitations of porous membranes, it is desirable to have an alternative method of providing conductive paths between microchannels in a microfluidic device. It would be particularly desirable if the conductive paths could be provided in a way that does not require the addition of an extra layer of material, such as the above-described layer of a porous membrane material, to the microfluidic device structure. Furthermore, it would be desirable that the dimensions of the conductive paths be able to be precisely and accurately defined. It would also be desirable that the degree of conductivity between channels be controllable. In its various aspects, embodiments of the present invention provide these and other advantages over currently known methods of allowing current to flow between the channels of a microfluidic device.